Increasingly higher demands are made on the technical cleanliness of the surfaces of dirt-sensitive components, units, products, or work or contact surfaces. Car manufacture, aeronautics, fine mechanics, semiconductor manufacture, food processing, health care or pharmaceutics should here be mentioned by way of example. For instance in the case of drives, fuel injection systems, brake systems, or other complex components and fluid systems, miniaturization and performance enhancement of the individual components and complex subassemblies made therefrom entail an increasing proneness to particle contamination from the production process
Therefore, the so-called “residual dirt” is sensed by way of measuring methods and documented for specifying product and component surfaces with respect to their technical cleanliness.
It often happens that the surfaces to be sampled are too large, of a complicated shape, rough or hardly analyzable in direct way for other reasons. That is why the particles adhering to the surface are normally removed by means of a cleaning solution or an adhesive film and are subsequently analyzed in the cleaning solution, in the filter residue or on the adhesive film with respect to their size, distribution and chemical nature. The total contamination is deduced from the analysis.
As a rule, a particle analysis comprises an evaluation of the particle characteristics with respect to number, size and particle type, sometimes also the morphology of the particles. To reduce the amount of time spent and to ensure a high reproducibility of the measurement results and a dirt input from the operating personnel that is as small as possible, this analysis is preferably carried out in an automated way.
A method and an apparatus for the automatic microscopic analysis of a particle accumulation in planar distribution, for instance on a filter, is known from U.S. Pat. No. 5,655,029 A. This document suggests that a sample including particles should be scanned automatically grid by grid by means of a microscope and the image of the measurement areas should be displayed on a monitor. The resulting images are digitized and evaluated with respect to size and morphology of the particles.
Especially metallic particles pose difficulties in the automated evaluation of such images. A micrograph of a metallic particle typically shows reflecting and non-reflecting areas and contains corridors with a similar brightness as the background. An automatically operating analysis system therefore runs the risk of not detecting such particles as a whole, but of assigning the image to several smaller particles.
To exclude such measurement errors, automatic particle counting systems are equipped with a polarizer by means of which the metallically reflecting particle regions can be masked and thus metallic particles also appear as a whole. These measures, however, have the drawback that the metallic nature of the particle can optically no longer be detected immediately. However, it is particularly the presence of the metallic particles that is of interest in the determination of the residual dirt.
US 2005/259861 A deals with the nondestructive inspection of metallic components by an optical analysis of the surface. To verify that a detected structure is a crack, an image of the surface is evaluated under polarized-light illumination with the help of a color camera on the basis of a calibration in different colors.
WO 94/14049 A1 describes the analysis of particles in a liquid stream, e.g. lubricating oil, for monitoring the wear situation of mechanical components in contact with one another. The liquid to be analyzed is here stopped at a point of measurement, so that it is present in the form of a thin film. With the help of a microscope equipped with a camera, optical filters and polarizers, one or several photographs are taken at the point of measurement and evaluated.